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1、1if 引导的条件句的用法 1)当 if 用于表示普遍真理和科学事实的条件句中，其主从句的谓语动词一般都用一般现在时。If you heat ice,it melts.如果给冰加热，冰便融化了。If you pour oil on water,it floats.把油倒在水上，油浮于水面。If the temperature drops to zero degrees centigrade,water freezes.如果温度降至零度，水便结冰了。If you divide eight by two,you get four.8 除以 2，等于 4。2)当 if 用于表示现在习惯动作条件句中，
2、其主从句的谓语也都用一般现在时。lf I make a promise,I keep it.我只要许下诺言，就信守不渝。if it rains,I go to work by bus.每逢雨天，我就搭乘公共汽车上班。3)当 if 用于表示过去习惯动作的条件句中，其主从句谓语动词都用一般过去式。lf I made promise,I kept it.以前我只要许下诺言，就总是信守不渝。If it rained,I went to work by bus.以前每逢下雨，我就坐公共汽车上班。4)if 还常与从句一般现在时，主句用 will/shall/can 加动词原形结构搭配使用。If you g
3、o to England,you will have to learn English.如果你到英国去，就必须学英语。If it is fine tomorrow,we can have a picnic somewhere.如果明天晴天，我们可以去野餐。5)if 从句还常出现以下结构，主句祈使句，从句一般现在时。If you wake up before me,give me a call.如果你比我醒得早，就叫醒我。If you meet Mary in London,introduce youself to her.如果你在伦敦遇见玛丽，就向她做自我介绍。6)有时 if 从句中还使用“s
4、hould+动词原形”结构，加重语气。If we should miss the 10 oclock train,we wont get there till after lunch.如果我们错过 10 点钟的火车，就要在午餐后才能到达。If he should fail to come,ask Philip to work in his place.万一他不来，就请菲利浦代替他工作。7)if 从句中也常使用“will+动词原形”结构，表示“意愿”。If he will accept the nomination,a lot of electors will vote for him.如果他愿
5、意接受提名，许多选民都会投他的票。lf you will wait for a few more minutes,the doctor will see you without your making an appointment.如果你愿意再等几分钟，医生不用预约就来看你。8)if 主从句中都用“would+动词原形”结构，表示“意愿”，用于客气的请求。If you would try Italian food,you would like it.如果您愿意尝试意大利食品，你会喜欢的。If you would read more carefully,you would understand
6、what the writer is trying to say.如果你愿意更加细心阅读，你会理解作家的意图。2If you offered me six times what you have just offered,I would still take my pound of flesh.即使你愿意给我六倍于你刚才提出的钱数，我仍然要拿我应得的那一磅肉。引导从句的连词if=even if，作“即使”解，引导的是让步状语从句，此句中的谓语动词用了虚拟语气。例如：If I am mistaken,you are mistaken,too.就算我错了，你也错了。If he did say that,Im sure he didnt intend to hurt your feelings.就算他真的那样说，我相信他也无意伤害你的感情。I will do it(even)if it takes me all the afternoon.即使要花费我一个下午的时间，我还是要坚持这件事。